The present invention relates to a process for the precipitation of a substantially iron-free chromium(III) compound at an elevated temperature and pressure from an acidic aqueous solution which contains iron and chromium.
Chromium chemicals are nowadays produced industrially only from chromite or ferrochromium by means of oxidizing calcination. Alkalic chromate is separated from the calcinated product by leaching it in water, and after several purification and washing stages, dichromate conversion is carried out using sodium bisulfate derived from the production of chromium trioxide, sulfuric acid, carbon dioxide or the like. Other chromium salts, such as chromates of potassium, ammonium, zinc and lead, chromium trioxide, Cr.sub.2 O.sub.3, Cr(OH)Cl.sub.2, CrO.sub.2, etc., and metallic chromium are also obtained from the purified alkali chromate solution directly or by a suitable treatment of alkali dichromate.
The previously known methods are both complicated and expensive. Since hexavalent chromium is toxic, these prior known processes have always involved environmental and health hazards.
The object of the present invention is to eliminate the above-mentioned disadvantages and to provide a process for the precipitation of a substantially iron-free chromium(III) compound, without coprecipitation of iron, at an elevated temperature and pressure from an acidic aqueous solution which contains iron and chromium. In the process according to the invention, the raw material used can be chromium-low or chromium-high chromite, ferrochromium, chromium-bearing scrap, etc.